As was pointed out, it might be better to check using length instead of defined. Remember that zero is false, so a straight logic check will not tell you if it is supplied but zero. Consider:

unless ($query->param('is_a_chicken')) # Fails on 0
{
     print "You must choose an option.<BR>\n";
}
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As your appropriate values might be 0 or 1, this is going to fail on a valid submission. Better to have:

unless (length $query->param('is_a_chicken'))
{
     print "You must choose an option.<BR>\n";
}
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Though length does tend to complain about being forced to deal with undefined strings. This will lead to the more verbose but equally more robust version:

my $is_a_chicken = $query->param('is_a_chicken');
unless (defined ($is_a_chicken) && length ($is_a_chicken))
{
     print "You must choose an option.<BR>\n";
}
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Of course, if you were doing this sort of thing all the time you could make a quick helper method to do it quickly:

sub provided
{
     return defined($_[0]) && length ($_[0]);
}
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Which would simplify as follows:

unless (provided ($query->param('is_a_chicken'))
{
     print "You must choose an option.<BR>\n";
}
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